Impact amelioration system for nuclear fuel storage

ABSTRACT

An impact amelioration system for nuclear fuel storage components in one embodiment includes a fuel storage canister and outer cask receiving the canister. The canister is configured for storing spent nuclear fuel or other high level radioactive waste. A plurality of impact limiter assemblies are disposed on the bottom closure plate of the cask at the canister interface. Each impact limiter assembly comprises an impact limiter plug frictionally engaged with a corresponding plug hole formed in the cask closure plate. The canister rests on tops of the plugs, which may protrude upwards beyond the top surface of the bottom closure lid. The plugs and holes may mating tapered and frictionally engaged surfaces. During a cask drop event, the canister drives the plugs deeper into the plug holes and elastoplastically deform to dissipate the kinetic impact energy and protect the structural integrity of the canister and its contents.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/954,083 filed Dec. 27, 2019, which is incorporated herein byreference in its entirety.

BACKGROUND

The present invention relates generally to systems and vessels forstoring high level radioactive waste such as used or spent nuclear fuel(SNF), and more particularly to an improved system which ameliorates theeffects of a forceful impact on such nuclear fuel storage vessels andconcomitantly the SNF assemblies stored therein.

In the operation of nuclear reactors, the nuclear energy source is inthe form of hollow Zircaloy tubes filled with enriched uranium,collectively arranged in multiple assemblages referred to as fuelassemblies. When the energy in the fuel assembly has been depleted to acertain predetermined level, the used or “spent” nuclear fuel (SNF)assemblies are removed from the nuclear reactor. The standard structureused to package used or spent fuel assemblies discharged from lightwater reactors for off-site shipment or on-site dry storage is known asthe fuel basket. The fuel basket is essentially an assemblage ofprismatic storage cells each of which is sized to store one fuelassembly that comprises a plurality of individual spent nuclear fuelrods. The fuel basket is arranged inside a cylindrical metallic fuelstorage canister, which is often referred to as a multi-purpose canister(MPC) that forms the primary nuclear waste containment barrier. SuchMPCs are available from Holtec International of Camden, N.J. The fuelassemblies are typically loaded into the canister while submerged in thespent fuel pool of the reactor containment structure to minimizeradiation exposure to personnel.

The fuel canister loaded with SNF (or other high level radioactivewaste) is then placed into an outer overpack or cask, which forms thesecondary containment, for safe transport and storage of the multiplespent fuel assemblies. Casks are heavy radiation shielded containersused to store and/or transfer the SNF canister from the spent fuel pool(“transfer cask”) in the nuclear reactor containment structure to a moreremote staging area for interim term storage such as in the dry caskstorage system of an on-site or off-site independent spent fuel storageinstallation (ISFSI) until a final repository for spent nuclear fuel isavailable from the federal government.

Drop events involving heavy loads such as nuclear waste fuel casks areamong the more serious accidents in industry. In the nuclear industry,an accidental drop of a cask onto a stationary reinforced concretesurface is a typical postulated scenario involving a hard and heavyobject slamming onto a highly inflexible surface. Classical dynamicsteaches us that the deceleration g-load under such an impact scenario isroughly proportional to the square root of the stiffness of theimpacting interface. The more rigid the impactor and the stationarytarget, the higher is the g-load. Reducing the g-load is essential tominimize the physical damage to the colliding bodies; which iscritically important if one of the two bodies contains a hazardousradioactive material such as spent nuclear fuel.

Accordingly, there remains a need for improvements in controlling andreducing the g-load associated with impacts occurring with the foregoingnuclear waste storage systems.

BRIEF SUMMARY

The present application discloses an impact amelioration or limitingsystem usable in nuclear waste fuel storage vessels. The system operatesto ameliorate and reduce the g-load or force (gravitational) imparted tosuch vessels due to mutual impact between the vessels resulting from adrop event. The proposed impact limiting system design can compriseinstalling one or preferably more tapered impact limiter rods or plugsin closely fitting and frictionally engaged tapered plug holes formed inone of the two mutually impacting vessels. The combination tapered plugand corresponding hole collectively defines an impact limiter assembly.In one embodiment, the impacting vessels may be without limitation anouter nuclear waste transfer overpack or cask and a SNF storage canister(aka fuel canister) such as a MPC described above. The impact limiterrods or plugs and corresponding tapered plug holes may be arranged onthe cask in one configuration at the interface between the bottom of thecanister and bottom closure plate of the cask. The impact ameliorationsystem is designed to absorb and dissipate at least a portion of thekinetic energy imparted to the vessels during a cask drop event, asfurther described herein.

The impact limiter plugs are partially embedded in their respective plugholes. Under impact during a generally vertical drop scenario, eachtapered impact limiter plug that may be provided when acted upon by thecanister will advance a distance deeper inside its respective taperedhole in the cask. The impact force of the plug's kinetic energy isabsorbed by the combined action of interfacial friction (between engagedside surfaces of the plug and hole walls) and the elastic-plastic(elastoplastic) deformation and expansion of the plugs within thetapered holes. Accordingly, the partially embedded plugs which protrudeabove top surface of the bottom closure plate of the cask are drivendeeper into the plug holes by the impact force. Calculations show that asuitable choice of the principal parameters such as the material of thetapered rod, angle of taper, rod diameter, and number of impact limiterrods or plugs provided results in reducing the peak g-load resultingfrom the impact significantly. Advantageously, this protects andminimizes or prevents the spent nuclear fuel (SNF) assemblies storedwithin the fuel canister from damage during the impact scenario.

A plurality of impact limiter rod or plugs and corresponding taperedplug holes may be arrayed around and partially embedded in the topsurface of the bottom closure plate of the cask. The plugs protrudeupwards beyond the top surface towards the canister in a patternselected to provide impact protection in a uniform manner at the bottomor lower cask to canister interface. The canister is seated on the topsurfaces of the plugs which act as pedestals that support the canisterin a spaced apart manner from the cask bottom closure plate. Thecanister therefore does not directly contact the bottom closure plate ofthe cask. All quadrants of the cask bottom closure plate may include atleast one impact limiter assembly (i.e. tapered plug and hole), butpreferably multiple impact limiter assemblies. This ensures evendistribution of the impact forces in the event of a generally straightvertical drop and/or guarantees that an off-center drop at an angle willresult in at least some impact limiter assemblies being positioned toabsorb the resultant impact forces and decelerate the canister to reducepeak g-loads.

An impact amelioration system for nuclear fuel storage components in oneembodiment comprises: a fuel storage canister comprising a first shellextending along a vertical centerline, the canister configured forstoring nuclear fuel; an outer cask defining a cavity receiving thecanister, the cask comprising a second shell and a bottom closure plateattached to the second shell; a plurality of impact limiter assembliesdisposed on the bottom closure plate at a canister to cask interface,each of the impact limiter assemblies comprising a plug frictionallyengaged with a corresponding plug hole formed in the bottom closureplate; wherein the plugs engage the canister.

A method for ameliorating impact between components of a fuel storagesystem in one embodiment comprises: partially embedding a plurality ofimpact limiter plugs in corresponding plug holes formed in a bottomclosure plate of a cask; seating the canister on the plugs, the plugsbeing positioned at a first depth in the plug holes; impacting thecanister against the plugs with an impact force; and driving the plugsto a second depth in the plug holes deeper than the first depth.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein likeelements are labeled similarly and in which:

FIG. 1 is a front cross-sectional perspective view of an impactamelioration system for nuclear fuel storage according to the presentdisclosure including a transfer cask and fuel canister;

FIG. 2 is a side cross sectional view thereof;

FIG. 3 is an exploded view thereof;

FIG. 4A is a front elevation view thereof;

FIG. 4B is a detail taken from FIG. 4A;

FIG. 5 is a partial bottom view of the cask;

FIG. 6 is a partial top view of the cask;

FIG. 7 is a top perspective view of the bottom closure plate of thecask;

FIG. 8 is a side cross-sectional view of the bottom closure plate;

FIG. 9A is a side cross-sectional view showing an impact limiterassembly of the system comprising an impact limiter plug and mating plughole shown in FIGS. 1-4B;

FIG. 9B is a side cross-sectional thereof showing the plug in aninstalled pre-impact position;

FIG. 9C is a side cross-sectional thereof showing the plug in a deeperpost-impact position in the plug hole after application of an impactforce resulting from a cask drop event;

FIG. 10 is a cross-sectional perspective view of the cask bottom closureplate showing a second embodiment of a impact limiter assembly;

FIG. 11 is a detail taken from FIG. 10 ;

FIG. 12 is a cross-sectional perspective view of the cask bottom closureplate showing a third embodiment of the impact limiter assembly;

FIG. 13 is a detail taken from FIG. 12 ; and

FIG. 14 is a perspective view of an exemplary nuclear fuel assembly ofthe type which may be stored in the canister.

All drawings are schematic and not necessarily to scale. Features shownnumbered in certain figures which may appear un-numbered in otherfigures are the same features unless noted otherwise herein. A generalreference herein to a figure by a whole number which includes relatedfigures sharing the same whole number but with different alphabeticalsuffixes shall be construed as a reference to all of those figuresunless expressly noted otherwise.

DETAILED DESCRIPTION

The features and benefits of the invention are illustrated and describedherein by reference to non-limiting exemplary (“example”) embodiments.This description of exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. Accordingly, the disclosureexpressly should not be limited to such exemplary embodimentsillustrating some possible non-limiting combination of features that mayexist alone or in other combinations of features.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivatives thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

As used throughout, any ranges disclosed herein are used as shorthandfor describing each and every value that is within the range. Any valuewithin the range can be selected as the terminus of the range. Inaddition, any references cited herein are hereby incorporated byreference in their entireties. In the event of a conflict in adefinition in the present disclosure and that of a cited reference, thepresent disclosure controls.

The terms “seal weld or welding” if used herein shall be construedaccording to its conventional meaning in the art to be a continuous weldwhich forms a gas-tight hermetically sealed joint between the partsjoined by the weld.

FIGS. 1-13 depicts various aspects of an impact amelioration or limitersystem associated with nuclear waste storage systems comprising vesselsused in the storage of spent nuclear fuel (SNF) or other irradiated highlevel radioactive waste materials removed from the nuclear reactorcontainment. The amelioration system generally comprises an outertransfer overpack or cask 100 and a waste fuel (e.g. SNF) canister 120configured for storage inside the cask. Features of each storage vesseland the impact amelioration system will now be further described.

Canister 120 may be used for storing any type of high level radioactivenuclear waste, including without limitation spent nuclear fuel (SNF) orother forms of radioactive waste materials removed from the reactor. TheSNF or simply fuel canister for short may be any commercially-availablenuclear fuel/waste storage canister, such as a multi-purpose canister(MPC) available from Holtec International of Camden, N.J. or other.

Canister 120 has a vertically elongated and metallic body comprised of acylindrical shell 121 extending along a vertical centerline Vc whichpasses through the geometric center of the shell. Canister 120 includesa bottom baseplate 122 seal welded to a bottom end of the shell, and anopen top 126 which may be closed by an attached lid 125 (schematicallyshown in dashed lines in FIG. 3 to avoid obscuring other aspects of theimage). Lid 125 may be seal welded to a top end 126 of the canistershell 121 to form a hermetically sealed cavity 127 inside the canister.The foregoing canister parts may be formed of any suitable metal, suchas for example without limitation steel including stainless steel forcorrosion protection.

Fuel basket 123 is disposed in cavity 127 of the canister 120 and isseated on the bottom baseplate 122 as shown. The fuel basket may bewelded to the baseplate for stability in some embodiments. In someembodiments, the baseplate 122 may extend laterally outwards beyond thesides of the fuel basket 123 around the entire perimeter of the fuelbasket as shown.

The fuel basket 123 is a honeycomb prismatic structure which in oneembodiment may be formed by a plurality of interlocked and orthogonallyarranged slotted plates 123 a built up to a selected height invertically stacked tiers. The plates of fuel basket 123 define a gridarray of plural vertically-extending openings forming fuel assemblystorage cells 124. Each cell is configured in cross-sectional area andshape to hold a single U.S. style fuel assembly 28, which containsmultitude of spent nuclear fuel rods 28 a (or other nuclear waste). Anexemplary fuel assembly of this type having a conventional rectilinearcross-sectional configuration is shown in FIG. 14 . Such fuel assembliesand the foregoing fuel basket structure are well known in the industry.The open cells 124 of the fuel basket are defined by the orthogonallyintersecting slotted plates 210, and therefore have a concomitantlyrectilinear cross-sectional shape (e.g. square). This gives the fuelbasket an overall compound rectilinear polygonal shape in transversecross section as shown which includes multi-faceted and stepped exteriorperipheral side surfaces collectively defined by the flat lateralperipheral sidewalls of the outermost exterior slotted plates 123 a.

Transfer cask 100 has a vertically elongated metallic body including acylindrical shell 101, circular top closure plate 102 attached to thetop end of the shell, and a circular bottom closure plate 103 attachedto the bottom end of the shell. A top ring plate 107 may be providedwhich is fixedly attached to the top end of shell 101 such as viawelding. A bottom ring plate 106 may be fixedly attached (e.g. sealwelded) to the upper or top surface 105 of the bottom closure plate 103at its periphery; which ring plate in turn is fixedly attached (e.g.seal welded) to the bottom end of the shell 101. The top closure plate102 may also be seal welded to the shell 101, or in some embodiments mayinstead be bolted and gasketed to the cask instead to provide easieraccess to the canister 120. An internal cavity 104 is defined by thecask which extends for a full height of the cask. The cavity 104 isconfigured in dimension and transverse cross-sectional area to hold onlya single fuel canister 120 in some embodiments as is conventionalpractice in the art.

The circular bottom closure plate 103 of cask 100 may be consideredsomewhat cup-shaped in one embodiment in view of the raised bottom ringplate 105 which rises up a short distance above the horizontal flat topsurface 105 of the bottom closure plate. This construction defines arecessed canister seating area 108 which helps center and stabilize thecanister 120 when loaded into the cask. The bottom baseplate 122 ofcanister 120 is at least partially received in the recessed canisterseating area as shown in FIGS. 1 and 2 .

The cask 100 is a heavy radiation shielded storage vessel. Thecylindrical shell 101 of cask 100 forms a sidewall which may have acomposite construction including an outer shell member 109, inner shellmember 110, and radiation shielding material(s) 111 disposed between theshell members. In some embodiments, the shielding material 110 maycomprise concrete, lead, boron-containing materials, or a combination ofthese or other materials effective to block and/or attenuate gamma andneutron radiation emitted by the nuclear waste (e.g. fuel assemblies)stored in canister 120 when loaded into the cask 100. Any suitabletypes, thicknesses, and arrangement of shielding materials may be usedto provide the necessary degree of shielding.

The outer and inner shell members 109, 110 of the cylindrical shell 101of cask 100 may be formed of a suitable metal such as steel. The top andbottom closure plates 102, 103, and the top and bottom ring plates 107,106 may similarly be formed of metal such as steel.

In conventional cask construction and deployment, the canister is seateddirectly onto the bottom closure plate of the cask 100 in an abuttingrelationship. A flat to flat interface is formed between the entirety ofthe bottom baseplate of the canister and the bottom closure plate of thecask. In the event the cask with canister loaded therein is dropped ontoan immovable/stationary hard surface (e.g. top of concrete slab 115 orother relatively hard/compacted material) as shown in FIG. 1 , there isno impact protection for the canister which might decrease the g-load orforce resulting from the impact force of the cask striking the surface.The kinetic energy of the resultant impact force generated by the dropis transmitted through the bottom closure plate of the cask directly tothe baseplate of the canister and then to fuel assemblies therein, whichtypically rest directly on the baseplate. The structural integrity ofthe nuclear fuel assemblies and SNF therein are therefore exposed todamage due to the unmitigated g-load or forces resulting from the dropevent.

The present disclosure provides an impact amelioration or limitingsystem configured to absorb and minimize the actual g-load/forcetransmitted through the cask 100 during a drop event to protect the fuelcanister 120. With continuing general reference to FIGS. 1-13 , theamelioration system may comprise a plurality of impact limiterassemblies arranged at the lower canister to cask interface (i.e. bottomof canister baseplate 122 to top of cask bottom closure plate 103).

In one embodiment with specific initial reference to FIGS. 1-9C, theimpact limiter assemblies 130 each comprise an impact limiter rod orplug 130 and a corresponding plug hole 140. Plug holes 140 may becomplementary configured to the plugs 131 in shape/profile. In oneembodiment, the sides of the plugs and plug holes may each be tapered.In one embodiment, the plugs 131 may have a frustoconical shape and atleast a portion of the plug holes 140 may have a complementaryfrustoconical shape. In the embodiment shown in FIGS. 9A-C, the entireplug hole 140 is frustoconical in shape from top to bottom.

The impact limiter plugs 131 may comprise a solid body including a topsurface 132, bottom surface 133, and sides 134 extending therebetween.The top surface may be flat and larger in surface area than the bottomsurface defining an overall wedge-shaped plug. The bottom surface 133may also be flat as shown and parallel to the top surface 132.Accordingly, sides 133 may be tapered having an angle of taper A1 whichdefines a plug body having a frustoconical shape as shown.

Plug holes 140 may be complementary configured to the plugs 131. Plugholes 140 comprise an open top 141 configured for at least partiallyreceiving and embedding the plugs 131 therein, a flat closed bottom 142formed by the cask bottom closure plate 103, and tapered sidewalls 143extending therebetween. The open top may have larger projected open areathan the closed bottom defined by bottom surface 144 of the plug holedefining a wedge-shaped hole. Accordingly, sidewalls 143 of plug hole140 may be tapered having an angle of taper A2 which defines a plug holehaving a frustoconical shape as shown. In certain embodiments, angle oftaper A2 of the plug holes 140 may be the same as the angle of taper A1of the impact limiter plugs 131. The plugs however may be have a maximumdiameter D1 defined by the top surface 132 which is slightly larger thanthe diameter D2 of the open top 141 of plug holes 140 such that theplugs cannot fully enter the plug holes and contact their bottomsurfaces 144 (see, e.g. FIG. 9B in the pre-impact embedment position ofthe plugs in the holes). The slight oversizing of the plugs 131 andmating tapers of the plugs and their associated plug holes 140 createfrictional engagement therebetween the mutually engaged plug sides 134and plug hole sidewalls which retains the plugs in position spacedvertically above from the bottom surface 144 of the plug holes. Thebottom surface 133 of plugs 131 may also be larger in diameter than thebottom surface 142 of the plug holes 140. Accordingly, the slightlylarger diameter plugs 131 are prevented from slipping completely intothe plug holes 140 to the bottom even though the angle of tapers A1, A2may be the same for each feature (see, e.g. FIG. 9B pre-impactfrictionally engaged position of plugs).

In certain exemplary embodiments, the angles of taper A1 and A2 of theplugs 131 and plug holes 140 respectively may be between 30 and 90degrees, and more preferably between 60 and 90 degrees. The angles oftaper A1 and A2 may be about 82 degrees (+/−3 degrees to account forfabrication tolerances) as one non-limiting example. Other suitabletaper angles may be used.

When the impact limiter plugs 131 are securely embedded in andfrictionally engaged with the plug holes 140 such that the plugs areretained and cannot easily be removed by hand (see, e.g. FIG. 9B), theupper portions of the plugs protrude upward above the top surface 105 ofthe cask bottom closure plate 103 as shown. Top surfaces 132 of theplugs 131 are therefore elevated above the closure plate 103 formingplateaus or pedestals which collectively act as a seating surface toengage and support the bottom baseplate 122 of the canister 120 in araised manner elevated above the top surface of the bottom closureplate. When the canister is positioned on the plugs 131, the canister istherefore spaced apart from the bottom closure plate 103 (i.e. topsurface 105 thereof) by a vertical space or gap G (see, e.g. FIG. 4B).The gap G advantageously provides a buffer or cushion zone allowing thecanister to gradually move downwards in the cask 100 as the plugs 131elastoplastically deform while moving deeper into the plug holes underthe kinetic impact forces generated by the cask striking a hard surfaceduring a drop event (see, e.g. FIG. 1 ). The impact limiter plugs 131deform and progress deeper in plug holes 140 due to the resultant impactforces (i.e. canister against the plugs) to decelerate the canistermotion and reduce the g-load which protects the canister 120 and fuelassemblies therein. This is demonstrated in the test example describedfurther below.

FIG. 9A shows a single impact limiter plug 131 positioned above andready for insertion/embedment in its mating plug hole 140. To installthe plug, the plug is loosely inserted and then partially driven intothe plug hole by a striking device such as a hammer or other deviceuntil the plug becomes snuggly fitted in and frictionally engaged withthe sidewalls 143 of the hole. This eliminates looseness of the plugswhile the canister 120 is loaded into the cask 100. The frictionally andmutually engaged tapers of the sides 134 of plugs 131 and plug holesidewalls 143 thus retain the fitted plugs in the holes via a frictionfit. The plugs therefore are not loosely placed in the plug holes, butrather cannot be removed by hand when properly installed. The plugs arenow partially embedded in their respective plug holes as shown in FIG.9B and ready for service to receive and seat the canister 120 thereonwhen loaded into the cask 100. In this pre-impact position shown, thebottom surface 133 of plug 131 is spaced vertically apart from thebottom surface 144 of the plug hole 140. This provides space for theplug to move deeper into the plug hole as the plug is forced inwardsinto the hole as it undergoes elastoplastic deformation due to impactforces generated by the drop event.

In the occurrence of a cask drop event (see, e.g. FIG. 1 ), the cask 100falls vertically for a distance and may strike/impact a hard surfacesuch as that defined by a. concrete pad/slab 115. This accident mayoccur if the cask rigging or hoist mechanism associated with atrack-driven cask crawler, which is commonly used in the industry forlifting/lowering and transporting the cask with fuel canister 120therein, were to fail. However, other scenarios of dropping the cask, ordropping canister into the cask while loading it therein, are possibleas well. The bottom closure plate 103 of the cask is the firstcontainment vessel to impact the immovable hard surface and decelerateto zero acceleration due to gravity. The momentum of the fallingcanister 120 inside the cask 103 resulting from the drop causes thecanister to continue its downward motion momentarily (e.g. fraction of asecond) until its movement is in turn fully arrested by engagement withthe impact limiter plug assemblies 130 on the bottom closure plate 103of cask 100. The baseplate 122 of the canister 100 may remain engagedwith the impact limiter plugs 131 during the fall or may slightly moveajar, depending on the height of the drop and relative weights of thecask and canister (cask typically being heavier due to its thicksidewalls which may include concrete for radiation shielding). In eitherevent, the impact force F (g-load/force) of the canister against theimpact limiter plugs 131 illustrated in FIG. 9B causes the plugs tobecome driven deeper into their respective plug holes 140 by overcomingthe interfacial frictionally engagement forces between the sides 134 ofthe plugs and corresponding hole sidewalls 143 and elastoplasticdeformation of the metallic plugs. This deeper second position of theplugs 131 in the holes 140 is shown in FIG. 9C. In this figure, thebottom surfaces 133 of the now more deeply embedded plugs after impact(“post-impact position) are separated from the bottom surface 144 of theplug holes by a lesser distance or space by comparison than the“pre-impact” plug position shown in FIG. 9B. Similarly, the tops of theimpact limiter plugs may still protrude upward beyond the top surface105 of the cask bottom closure plate 103, but also by a lesser amount ordistance than pre-impact. In some impact events scenarios andembodiments, the tops of the plugs may be driven completely flush withthe top surface of the bottom closure plate.

Due to the impact of the falling cask scenario (drop event), the plugs131 concomitantly undergo some degree of elastoplastic deformation asthey are driven deeper into their respective plug holes 140. In somecases depending on the angles of tapers A1, A2 and sizes used for theplugs and holes, and other parameters such as the metal materialselected for the plugs versus the cask bottom closure plate 103, theplugs may possibly contact the bottom surface 144 of the holes dependingon the magnitude of the kinetic impact force (which equates to theheight of drop). In some instances, the tops of the plugs may possiblydeform and mushroom due to the impact force which may reduce thepenetration depth of the plugs in the holes. In either case, thedeformation and frictional engagement of the plugs 131 with thesidewalls 143 of the plug holes 140 absorbs at least some of the impactforce and causes the canister 120 to more gradually decelerate, therebydecreasing the g-load imparted on the canister to better protect thestructural integrity of the canister and fuel assemblies stored therein.In sum, under impact, the tapered plugs 131 would advance inside thetapered holes 140 as the kinetic impact energy is dissipated by thecombined action of interfacial friction therebetween and theelastic/plastic expansion action or deformation of the plugs in the plugholes.

The principal engineering parameters of the impact amelioration systemsuch as the material selected for the tapered impact limiter plugs 131in contrast to the cask bottom closure plate 103 which defining thecorresponding plug holes 140, angle of taper A1 and A2 of the plugs andholes, plug diameter, and the number and pattern/arrangement of plugs onthe bottom closure plate make possible to decrease the peak g-loadimparted to the canister 120 during a cask drop event significantly.

In one non-limiting arrangement, a first group or cluster of impactlimiter plug assemblies 130 (pairs of tapered plugs 131 and mating plugholes 140) may be arranged in a circular array on the bottom closureplate 103 of the cask 100 (see, e.g. FIGS. 3-4 and 7-8 ). The plugassemblies are circumferentially spaced apart as shown. Depending on thediameter D1 of the plugs 131, additional circular arrays may be addedinside and/or outside of the array shown. In some embodiments, one ormore a center plug assemblies 130 may be located centrally with respectto and inside of the circular array. A single plug assembly located atand intersecting the vertical centerline Vc of the canister may beprovided in some embodiments. In other embodiments, a cluster of centerplug assemblies 130 may be provided and arranged in any suitable patternwithin the outer circular array of assemblies. The plug assemblies 130are located within the recessed canister seating area 108 of the caskbottom closure plate 103 inside the raise annular bottom ring plate 106as shown. This is the area which receives the bottom baseplate 122 ofthe fuel canister 120.

In other less preferred but possible embodiments contemplated, thearrangement of the plug assemblies 130 may be reversed to that shown.Accordingly, the plug holes 140 may be downward facing openings formedin the base plate 122 of canister 120 provided if the base plate issufficiently thick. The tapered the plugs 131 may be embedded in theholes and protrude downwards from the base plate to engage the topsurface of the cask bottom closure plate 103 when the canister is loadedtherein.

Test Example

To demonstrate the impact amelioration system concept, the case of afalling transfer cask 100 containing an MPC (canister 120) is consideredwith reference to FIG. 1 . The transfer cask is assumed to fall from aheight of 6.56 feet in this postulated scenario onto a reinforcedconcrete pad or slab 115. The following data characterizes thephysical/mechanical parameters of the computer simulated drop test:weight of transfer cask 100 body: 120,000 pounds; weight of the loadedMPC 120: 90,000 pounds; MPC diameter 75¾ inches; thickness of thetransfer cask baseplate 103: 5½ inches; Material of impact limiter rodor plug: ASME/ASTM SA479 stainless steel; Material of cask bottomclosure plate 103: ASME/ASTM SA516 Grade 70.

Calculations using LS-DYNA (a state-of-the-art impact dynamics codewidely used in the industry) showed the peak deceleration of the MPC tobe 262 g's when the transfer cask is dropped with the MPC restingdirectly onto the transfer cask baseplate without impact limiterassemblies 130. Next, using the present impact amelioration systemdisclosed herein, the cask's bottom closure plate 103 was equipped with16 circumferentially arranged impact limiting plugs 131 of 4-inchdiameter (D1) and 82 degree included angle of taper (A1) each situatedin frustoconical plug holes 140 also with 82 degree included angle oftaper (A2). An equal sized impactor at the centerline Vc of the MPC 120was also employed. When this second configuration with impact limiterassemblies 130 was employed, the peak deceleration of the MPC droppeddown to 180 g's. The impact limiter plugs 131 were driven into andadvanced in the holes by only 0.13 inch to achieve this substantialreduction in g-load. Therefore, by reducing the angle of taper in otherconfigurations, the penetration of the plugs 131 into the plug holes 140can be further increased, and the g-load correspondingly reducedfurther. Accordingly, the foregoing analysis demonstrates the benefitsof present impact amelioration system for reducing the g-load on thecanister and protecting the canister and fuel assemblies stored therein.

FIGS. 10 and 11 show an alternate embodiment of an impact limiterassembly. In this embodiment, the plug hole 150 includes an uppertapered portion 150 a similar to that previous described herein which isfrustoconical shaped. The adjoining lower portion 150 b of the plugholes 150 comprises sacrificial threads configured to deform under shearforces imparted by the plugs 131 when the plugs are driven deeper intothe plug holes under impact during a cask drop event. The plugs 131 havea mating threaded bottom extension 131 a engaged with the threaded hole.Shearing of the threads as the plug 131 is driven deeper into the plughole 150 after a cask drop event serves to extract impact energy fromthe fall. The deformation of sacrificial threads in conjunction with thefrictional forces acting between the plug and hole sidewalls mutuallycontribute and act in unison to absorb the g-forces acting on thecanister 100 during the drop event. The threaded lower portion 150 b ofthe plug holes 150 may extend complete through the bottom surface of thecask bottom closure plate 103, or in other embodiments may have a closesbottom which does not penetrate the bottom surface of the closure plate.Either embodiment may be used. It bears noting that the threaded impactlimiter plugs 131 also facilitate installation of the plugs by simplyrotating the plugs to threadably engage the threaded plug holes 150,thereby retaining the plugs until the canister 120 is loaded into thetransfer cask 100.

FIGS. 12 and 13 show yet another embodiment of an impact limiterassembly. In this embodiment, the plug hole 160 has straight sidewalls161 and a closed bottom. An annular expansion ring 170 is seated in plughole 160. Expansion ring 170 includes straight exterior sides 170 a anda vertical tapered central opening 171 of frustoconical shape which mayextend completely through the ring as shown. The opening 171 definescorresponding frustoconical walls which may be complementary configuredin angle of taper to the angle of taper A1 of the plug 131. The topsurface of the expansion ring 170 may be recessed within in plug hole160 below the top surface 105 of the cask bottom closure plate 103 asshown.

In this present embodiment of FIGS. 12 and 13 , impact limiter plug 131retains a frustoconical shaped central portion 135 but adds a radiallyprotruding peripheral flange 133 at the top of the plug as shown. Theplug with flange may have a diameter measured at its top surface(similar to diameter D1) which in this case is smaller than the topopening of the plug hole 160 such that the flange can at least enter theplug hole 160 as shown. The central portion 135 of plug 131 stillfrictionally engages the central opening 171 of the expansion ring 170to retain the plug in place in the pre-impact position shown.Preferably, the expansion ring 170 is sized in outer diameter so that asmall annular space is formed between the sides of the ring and thesidewalls 161 of plug hole 160. This provides room for the ring 170 toexpand under impact forces after a cask drop event.

In operation after the cask 100 is dropped, the impact limiter plug 131is driven deeper into tapered central opening 171. The impact force Facting on the mating tapered/angled surfaces of the plug and expansionring 170 within the central opening 171 has a lateral/horizontal forcecomponent (in additional to a vertical force component) as wellunderstood by those skilled in the art. The horizontally acting forcecomponent deforms and expands the ring radially outwards as it issqueezed between the plug 131 and plug hole 160 to close the annularspace between the ring and plug hole 160 sidewalls 161. In someinstances, the ring may possibly engage the sidewalls 161 as it radiallyexpands. The expansion ring in combination with mating tapered surfacesof the impact limiter plug 131 and expansion ring 170 act in unison toabsorb and reduce the g-load imparted to the canister 120 during thecask drop event. The peripheral flange 133 of plug 131 may completelyenter the plug hole 160. FIG. 13 shows the pre-impact position of theplug in the impact limiter assembly. Expansion ring 170 may be formed ofany suitable metallic or non-metallic material. Preferably, the ring isformed of a material having greater ductility (i.e. softer) than theplug 131 to facilitate the expansion of the ring. In one embodiment, theexpansion ring 170 is formed of metal such as steel or aluminum. Inother embodiments, the ring may be formed a non-metallic material suchas a dense polymer.

In view of all the foregoing embodiments of an impact ameliorationsystem, the included taper angles of the tapered plugs 131 and plugholes 140, their material of construction and dimensions, number andarrangement/pattern of impact limiter assemblies 130 on the cask bottomclosure lid 130, number and type of threads used in the embodiment ofFIGS. 10-11 , the height/thickness and material of the optionalexpansion ring 170 used in the embodiment of FIGS. 12-13 , and otheraspects are among the parameters that can be varied to obtain theoptimal energy extraction for a specific impact scenario to protect thecanister 120 and its waste fuel contents from severe damage.

The impact limiter plugs 131 can generally advance in the hole primarilyby expanding/deforming the plugs in an elastoplastic manner whichexceeds the yield stress of the material, and by overcoming the frictionat the tapered/angled interface between the plug and mating plug holes.The plugs are therefore preferably formed of a metallic elastoplasticmaterial such as without limitation steel which undergoes elastic andplastic deformation when the load/force exceeds the yield stress of thematerial. Plastic deformation beyond the yield stress connotes that theplug will retain permanent deformation and not return to its originalcondition (e.g. shape and dimensions). Depending on the materialselected for the cask bottom closure plate 103, the sidewalls of theplug holes may similarly undergo elastic-plastic deformation to absorbsome of the kinetic impact energy resulting from a cask drop event.

While the foregoing description and drawings represent some examplesystems, it will be understood that various additions, modifications andsubstitutions may be made therein without departing from the spirit andscope and range of equivalents of the accompanying claims. Inparticular, it will be clear to those skilled in the art that thepresent invention may be embodied in other forms, structures,arrangements, proportions, sizes, and with other elements, materials,and components, without departing from the spirit or essentialcharacteristics thereof. In addition, numerous variations in themethods/processes described herein may be made. One skilled in the artwill further appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, sizes, materials,and components and otherwise, used in the practice of the invention,which are particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being defined by the appended claims andequivalents thereof, and not limited to the foregoing description orembodiments. Rather, the appended claims should be construed broadly, toinclude other variants and embodiments of the invention, which may bemade by those skilled in the art without departing from the scope andrange of equivalents of the invention.

What is claimed is:
 1. An impact amelioration system for nuclear fuelstorage components comprising: a fuel storage canister comprising afirst shell extending along a vertical centerline, the canisterconfigured for storing nuclear fuel; an outer cask defining a cavityreceiving the canister, the cask comprising a second shell and a bottomclosure plate attached to the second shell; a plurality of impactlimiter assemblies disposed on the bottom closure plate at a canister tocask interface, each of the impact limiter assemblies comprising a plugfrictionally engaged with a corresponding plug hole formed in the bottomclosure plate; wherein the plugs engage the canister; wherein the plugsare tapered and the plug holes have a corresponding taper.
 2. The systemaccording to claim 1, wherein the plugs protrude upwards beyond thebottom closure plate to engage the canister.
 3. The system according toclaim 2, wherein the canister comprises a baseplate supported by andresting on the plugs which form pedestals.
 4. The system according toclaim 3, wherein the canister is separated from the bottom closure plateof the cask by a vertical gap.
 5. The system according to claim 3,wherein the plugs have a planar top surface which abuttingly engages thebaseplate of the canister via a flat-to-flat interface.
 6. The systemaccording to claim 1, wherein the plug holes have a closed bottom andopen top.
 7. The system according to claim 1, wherein an angle of taperof the plugs is equal to an angle of taper of the plug holes.
 8. Thesystem according to claim 1, wherein the plugs have a frustoconicalshape and at least a portion of the plug holes have a frustoconicalshape.
 9. The system according to claim 8, wherein the plug holes have afull frustoconical shape from top to bottom.
 10. The system according toclaim 1, wherein the plugs are configured to be driven deeper into theplug holes when impacted by the canister.
 11. The system according toclaim 10, wherein the plugs are movable between an upper shallowerposition with respect to the plug holes when not impacted by thecanister, and a lower, deeper position with respect to the plug holesafter impact by the canister.
 12. The system according to claim 1,wherein the impact limiter assemblies are arranged in a circular arrayon the bottom closure plate of the cask, the impact limiter assembliesbeing circumferentially spaced apart.
 13. An impact amelioration systemfor nuclear fuel storage components comprising: a fuel storage canistercomprising a first shell extending along a vertical centerline, thecanister configured for storing nuclear fuel; an outer cask defining acavity receiving the canister, the cask comprising a second shell and abottom closure plate attached to the second shell; a plurality of impactlimiter assemblies disposed on the bottom closure plate at a canister tocask interface, each of the impact limiter assemblies comprising a plugfrictionally engaged with a corresponding plug hole formed in the bottomclosure plate; wherein the plugs engage the canister; wherein the impactlimiter assemblies are arranged in a circular array on the bottomclosure plate of the cask, the impact limiter assemblies beingcircumferentially spaced apart; and further comprising at least onecenter impact limiter assembly located centrally inside the circulararray.
 14. The system according to claim 1, wherein the plugs are formedof an elastoplastic material which undergoes elastoplastic deformationto dissipate kinetic impact energy and protect the canister from a caskdrop event.
 15. The system according to claim 14, wherein the plugs areformed of a steel material and the bottom closure plate of the cask isformed of a steel material different than the plugs.
 16. The systemaccording to claim 1, wherein the plugs have a frustoconicalwedge-shape.